The present invention relates to an improved connection of blades on a rotor disc of a gas turbine.
As is known, gas turbines are machines which consist of a compressor and a turbine with one or more stages, wherein these components are connected to each other by a rotary shaft and wherein a combustion chamber is provided between the compressor and the turbine.
The gas output from the combustion chamber, which has a high temperature and a high pressure, reaches through corresponding pipes the different stages of the turbine, which transforms the enthalpy of the gas into mechanical energy available to a user.
In turbines with two stages the gas is processed in the first stage of the turbine in temperature and pressure conditions which are very high and undergoes a first expansion there.
Then, in the second stage of the turbine it undergoes a second expansion in temperature and pressure conditions which are lower than those used in the preceding stage.
It is also known that in order to obtain the maximum performance from a specific gas turbine it is necessary for the temperature of the gas to be as high as possible.
However, the maximum temperature values which can be obtained in use of the turbine are limited by the resistance of the materials which are used at present.
It is also known that in gas turbines the rotor blades do not form a single body with the rotor disc, but are retained by means of their base extensions in appropriate seats provided on the circumference of the disc.
In particular, the seats used at present have sides with a grooved profile, in which the end portion of the foot or root of the corresponding blade is engaged.
A problem which is particularly significant in the present art is therefore that of guaranteeing an optimal connection of the blades on the rotor disc, in all the conditions of functioning of the machine.
In fact it should be noted that the method of connection of the blades on the rotor disc represents a crucial aspect of the design of any rotor, taking into account the fact that the disc must withstand satisfactorily and reliably the loads generated by the blades without giving rise to breakages or other similar problems.
In fact it is known that during functioning of the machine, the rotor blades are subjected to high stresses both in the radial direction, and to a lesser extent in the axial direction.
The radial stresses are caused by the high speed of rotation of the turbine, whereas the axial stresses are caused by the effect produced by the flow of gas on the profiled surfaces of the blades.
The same flow of gas transmits to the blades the circumferential component of the stress which makes it possible to gather useful power at the drive shaft.
However, the method of connection of the blades must use the smallest possible dimensions, occupying truly limited spaces, such as to reduce the assembly constituted by the rotor disc and blades to the smallest possible dimensions.
Furthermore, nowadays, the trend is to obtain gas turbines with increasingly high performance levels.
This involves the fact of having to increase both the speed of rotation and the combustion temperature. There is consequently also an increase in the temperature of the gases which expand in the stages of the turbine against the blades.
In fact this gives rise to an increase in the stresses on the connection between the blades and rotor discs of the turbine, with increasingly great difficulty in guaranteeing an adequate service life of the blades and rotor discs.
At present the connection most widely used is that which is commonly known as the xe2x80x9cpine treexe2x80x9d type.
It consists of shaping the root or foot of the blade such that its cross-section assumes a characteristic shape which is reminiscent of an overturned pine tree.
In this particular form the sides of the root have a grooved profile such as to form a series of teeth with a rounded profile; otherwise, in its lower end the root is formed by the connection of the two lower teeth of the two sides.
These roots are connected to seats or coupling slots complementary to them which are provided on the circumference of the rotor disc, such that grooves in the sides of the seat correspond to the teeth of the root and a groove at the base of the seat corresponds to the lower end of the root.
In conventional embodiments, these seats for the roots of the blades extend in a direction which is substantially parallel to the axis of the rotor disc.
On the other hand in different embodiments, the seats for the roots extend substantially in a direction which is inclined relative to the axis of the disc itself.
This type of connection has areas of particular concentration of stress which can be determined more specifically as being at the bottom of the groove, on the base of the seat, and on the base of the grooves of each tooth, which constitutes the actual connection profile.
The main object of the present invention is thus to eliminate the above-described disadvantages and in particular to provide an improved connection for blades on a rotor disc of a gas turbine which makes it possible to reduce the concentrations of stress, thus making it possible to increase the speed of rotation of the machines or to increase the temperature of the fluid, or an appropriate combination of these factors.
Another object of the present invention is to provide an improved connection of blades on a rotor disc of a gas turbine, which permits easy assembly and dismantling, according to requirements, of the blades of the different stages of the turbine.
Another object of the present invention is to provide an improved connection of blades on a rotor disc of a gas turbine which is also highly reliable.
A further object of the present invention is to obtain a service life of the components which is far longer than that which can be obtained at present with the connections used.
A further object of the present invention is to provide an improved connection of blades on a rotor disc of a gas turbine which is particularly simple and functional, has relatively low costs, and can be produced by means of conventional processing.
According to the invention it has also been possible to determine that the reduction of the maximum values of the stresses in the areas of concentration of the forces gives rise to a considerable increase in the life of the components.